FIG. 1 schematically shows the connections of three phases of a conventional hard disk drive spindle motor. The motor comprises three windings WA, WB and WC, each having one terminal connected to a common center tap N. Three remaining terminals A, B and C of the three windings, respectively, are controlled by a three-phase switch bridge. Each of the terminals A, B and C is coupled to a low supply voltage GND through a low side switch SL, and is also coupled to a high supply voltage Vcc through a high side switch SH.
FIG. 2A shows exemplary voltage waveforms at the terminals A, B and C, illustrating a conventional control cycle for the switches SH and SL coupled to each of the terminals A, B, and C. Each of the terminals A, B and C is successively connected to the voltage Vcc when SH=1 and SL=0, and then is set at a high impedance or "tri-stated" when SH=0 and SL=0, and is finally connected to the low voltage GND when SH=0 and SL=1. As shown in FIG. 2A, the time intervals in which the terminals A, B, or C are connected to the voltage Vcc or to the low voltage GND are twice as long as the time intervals in which the terminals A, B, or C are tri-stated. Furthermore, the control cycles of the terminals B and C are phase shifted by 120.degree. and 240.degree., respectively, relative to the control cycle of the terminal A, so that, for each sixth of a full switching cycle, while one winding is in turn tri-stated, the two other windings are connected in series between the voltages Vcc and GND.
The voltage across the winding which is tri-stated is representative of the bemf of the motor. This voltage is measured and its zero-crossing, relative to the voltage at the center tap N, is detected to provide information of the speed of the motor which is used for servo-controlling the motor. As shown, the voltage at the terminal of a tri-stated phase begins with a spike due to inductive recirculation and continues with an increasing or decreasing portion of a sinusoid. The zero-crossing detection is enabled only during the sinusoidal portions.
In low power motors, the speed of the motor is adjusted by varying the conductance of the high side switches SH or the low side switches SL. This method of speed control causes a power loss in the switches which may be acceptable for low power motors but becomes excessive in higher power motors.
To reduce the power losses, the switches SH and SL are controlled to be switched on and off by a pulse-width modulation signal. However, control of the switches SH and SL by a pulse-width modulation signal affects the reliable detection of the bemf zero-crossings.
FIG. 2B illustrates an enlarged portion of the waveform shown in FIG. 2A, in a case where a pulse-width modulation signal is used to control the switches SH and SL. During the supply time interval of a pair of the windings, the voltage at one terminal of the pair of windings is chopped between the voltages Vcc and GND. When this terminal is tri-stated, its voltage is affected by the chopping which continues on the next terminal. As shown, the voltage at this terminal is chopped between the bemf value and a lower value. A false zero-crossing detection occurs at a time t.sub.1 well before the zero-crossing of the bemf which occurs at a time t.sub.0.
Some circuits, like circuit L6232B manufactured by SGS-Thomson Microelectronics, regulate the nominal speed of a motor by adjusting the conductance of the low side switches SL and, when the motor is started, the circuits use a pulse-width modulation signal in order to reduce the current drawn from the power supply. As indicated above, these circuits are only adapted to low power motors.